1. Field of the Invention
The present invention relates to beam-index type color cathode ray tube devices, and more particularly, to a color cathode ray tube device of the beam-index type in which a phosphor screen is provided with a plurality of color phosphor stripes accompanied with index phosphor on the inner surface of a screen panel portion facing a front panel portion and fluorescence from the index phosphor is detected by a photodetecting portion including a luminescence receiving plate member provided to face the outer surface of the screen panel portion.
2. Description of the Prior Art
There has been proposed a so-called reflex color cathode ray tube device of the beam-index type as one of the beam-index type color cathode ray tube devices which can be adapted generally to form color television receivers of relatively small size. The reflex color cathode ray tube device of the beam-index type comprises a flat color cathode ray tube having a front panel portion which allows colored light to pass therethrough and a screen panel portion which faces the front panel portion and is provided on the inner surface thereof with a phosphor screen including a plurality of color phosphor stripes of three primary colors and a plurality of index phosphor stripes, and a photodetecting portion disposed at the outside of the flat color cathode ray tube. The photodetecting portion includes a luminescence receiving plate member which is provided to face the outer surface of the screen panel portion of the flat color cathode ray tube for receiving fluorescence emitted from each of the index phosphor stripes and emitting secondary fluorescence in a wavelength range different from that of the fluorescence received and a fluorescence detector attached to the luminescence receiving plate member for detecting the secondary fluorescence emitted in the luminescence receiving plate member. With such a reflex color cathode ray tube device of the beam-index type, a color image displayed on the phosphor screen is observed through the front panel portion from the outside thereof and the fluorescence from each of the index phosphor stripes is detected substantially at the outside of the screen panel portion.
FIG. 1 shows a previously proposed reflex color cathode ray tube device of the beam-index type. This device is composed mainly of a color cathode ray tube 10 and a photodetecting portion 40 disposed at the outside of the color cathode ray tube 10.
The color cathode ray tube 10 has a flat glass envelope which includes a front panel portion 11, a screen panel portion 12 facing the front panel portion 11, and a neck portion 13 connected to both the front panel portion 11 and the screen panel portion 12. The screen panel portion 12 is provided on its inner surface with a phosphor screen 14 of rectangular configuration. The phosphor screen 14 includes a plurality of color phosphor stripes of three primary colors, namely, green, red and blue phosphor stripes and a plurality of index phosphor stripes. In the neck portion 13, an electron gun assembly 15 is contained to produce an electron beam impinging on the phosphor screen 14 to scan the same. The front panel portion 11 permits colored light to pass therethrough and the screen panel portion 12 permits index fluorescence emitted from each of the index phosphor stripes with a peak level at a range of ultraviolet rays in its spectral characteristic to pass therethrough. At a partial area 14s of the phosphor screen 14 with which each horizontal beam scanning starts, color phosphor stripe is not provided but the index phosphor stripes are provided.
The photodetecting portion 40 is composed of a luminescence receiving plate member 41 which is rectangular in shape and disposed to face the outer surface of the screen panel portion 12 and to extend along the phosphor screen 14, and an index fluorescence detector 42 which contains a photosensitive device such as a photodiode and is attached to a center of a side portion 41z of the luminescence receiving plate member 41 which extends in parallel with a top end 14z of the phosphor screen 14.
The luminescence receiving plate member 41 is formed of, for example, acrylic resins in which specific phosphors, which are excited by the index fluorescence from the index phosphor stripe and emit secondary index fluorescence in a wavelength range different from that of the index fluorescence received thereby and suitable for detection by the index fluorescence detector 42, are dispersed. This luminescence receiving plate member 41 is operative to receive the index fluorescence emitted by each of the index phosphor stripes to enter thereinto through the screen panel portion 12 and to produce the secondary index fluorescence in response to the index fluorescence received thereby.
In such a beam-index type color cathode ray tube device, when the phosphor screen 14 on the inner surface of the screen panel portion 12 is scanned by the electron beam generated by the electron gun assembly 15, the index fluorescence emitted by each of the index phosphor stripes enters into the luminescence receiving plate member 41 through the screen panel portion 12 and the secondary index fluorescence is produced in the luminescence receiving plate member 41 in respomse to the index fluorescence received thereby to be detected by the index fluorescence detector 42. Then, an index signal is obtained from the index fluorescence detector 42 to be used for causing the electron beam generated by the electron gun assembly 15 to be modulated in density with a color video signal supplied to the electron gun assembly 15 appropriately in response to momentary scanning positions of the electron beam on the phosphor screen 14. With such electron beam scanning, the green, red and blue phosphor stripes on the phosphor screen 14 emit green, red and blue fluorescences each having intensity determined in accordance with the density of the electron beams, respectively, and a color image formed by these green, red, and blue fluorescences is observed through the front panel portion 11 from the outside of the same.
Generally, a photosensitive device such as a photodiode has sensitivity to light which varies in response to changes in incidence angle of light entering thereinto in such a way that the larger the incidence angle the lower the sensitivity. Meanwhile, in the photodetecting portion 40 of the previously proposed reflex color cathode ray tube device of the beam-index type shown in FIG. 2, the luminescence receiving plate member 41 is shaped rectangularly in accordance with the phosphor screen 14 and the index fluorescence detector 42 is positioned at the center of the side portion 41z of the luminescence receiving plate member 41 extending in parallel with the top end 14z of the phosphor screen 14, and therefore the incident angle of the secondary index fluorescence to the index fluorescence detector 42 varies to a considerable degree in response to the shift of a location in the luminescence receiving plate member 41 from which the secondary index fluorescence is emitted. The result is that the index fluorescence detector 42 has sensitivity to the secondary index fluorescence which varies to a considerable degree in response to change in the location in the luminescence receiving plate member 41 from which the second index fluorescence is emitted. Especially, the secondary index fluorescence emitted from the corners of the luminescence receiving plate member 41 close to the side portion 41z of the same has a large incident angle to the index fluorescence detector 42, and accordingly the index fluorescence detector 42 has a low sensitivity to the secondary index fluorescence emitted from the corners of the luminescence receiving plate member 41 close to the side portion 41z of the same. In addition, at the corners of the phosphor screen 14, the electron beam scanning the phosphor screen 14 forms a relatively large landing spot and therefore each of the index phosphor stripes receives the electron beam with reduced intensity. This results in the index fluorescence enitted by the index phosphor stripe disposed at the corners of the phosphor screen 14 being reduced in intensity and consequently the secondary index fluorescence emitted in the corners of the luminescence receiving plate member 41 also being reduced in intensity.
Accordingly, in the previously proposed reflex color cathode ray tube devices of the beam-index type, the index signal obtained from the index fluorescence detector 42 varies in level to a considerable degree in response to the location on the phosphor screen 14 at which the index fluorescence is emitted by the index phosphor stripe. Especially, when the index fluorescence is emitted from the corner of the phosphor screen 14 close to the top end 14z, the level of the index signal is lowered considerably, and it is feared that, under the control with the index signal having an insufficient level, the electron beam for scanning the phosphor screen 14 is not modulated in density with the color video signal supplied to the electron gun assembly 15 appropriately and stably in response to the momentary scanning positions of the electron beam on the phosphor screen 14. Further, in the case of the electron beam for exciting each of the index phosphor stripes being increased in density in order to raise the intensity of the index fluorescence entering into the luminescence receiving plate member 41, a dark electron beam current in the cathode ray tube 10 is increased and therefore a black level of a color image on the phosphor screen 14 rises so as to deteriorate quality of the color image.
Then, for the purpose of avoiding the above mentioned problems or disadvantages that occur in the previously proposed reflex color cathode ray tube device of the beam-index type shown in FIGS. 1 and 2, it is considered, as shown in FIG. 3, to provide a pair of index fluorescence detectors 43 and 44 at right and left sections of the side portion 41z of the luminescence receiving plate member 41, respectively, and to synthesize both output signals obtained from the index fluorescence detectors 43 and 44 so as to produce an index signal. In the reflex color cathode ray tube device of the beam-index type provided with such index fluorescence detectors 43 and 44, secondary index fluorescence which is emitted from a location in the luminescence receiving plate member 41 corresponding to the upper left corner of the phosphor screen 14 at which the index fluorescence is emitted has a reduced incident angle to the index fluorescence detector 43. Similarly, secondary index fluorescence which is emitted from a location in the luminescence receiving plate member 41 corresponding to the upper right corner of the phosphor scree 14 at which the index fluorescence is emitted has also a reduced incident angle to the index fluorescence detector 44, compared with the incident angle to the index fluorescence detector 42 of the secondary index fluorescence emitted from a location in the luminescence receiving plate member 41 corresponding to one of upper left and right corners of the phosphor screen 14 at which the index fluorescence is emitted in the previously proposed device shown in FIGS. 1 and 2. Variations in incident angle to each of the index fluorescence detectors 43 and 44 of the secondary index fluorescence, which are caused in response to the shift of a location in the luminescence receiving plate member 41 from which the secondary index fluorescence is emitted, are reduced, compared with the variations in incident angle to the index fluorescence detector 42 of the seondary index fluorescence in the previously proposed device shown in FIGS. 1 and 2. Consequently, variations in level of the index signal obtained by synthesizing the detection output signals from the index fluorescence detectors 43 and 44, which are caused in response to the shift of a location on the phosphor screen 14 at which the index fluorescence is emitted, are also reduced, compared with those in the previously proposed device shown in FIGS. 1 and 2.
However, even though the photodetecting portion 40 is composed of the luminescence receiving plate member 41 and the index fluorescence detectors 43 and 44 are positioned in a manner as shown in FIG. 3, the secondary index fluorescence emitted from the locations in the luminescence receiving plate member 41 corresponding to the upper left and right corners of the phosphor screen 14, at which the electron beam scanning the phosphor screen 14 forms a relatively large landing spot and therefore each of the index phosphor stripes emits the index fluorescence with reduced intensity, still does not have a sufficiently reduced incident angle to the index fluorescence detector 43 or 44, and the variations of the incident angle to the index fluorescence detector 43 or 44 of the secondary index fluorescence, which are caused in response to the shift of the location in the luminescence receiving plate member 41 from which the secondary index fluorescence is emitted, are not reduced enough. Accordingly, the variations in level of the index signal obtained by synthesizing the detection output signals from the index fluorescence detectors 43 and 44, which are caused in response to the shift of the location on the phosphor screen 14 at which the index fluorescence is emitted, are not reduced enough, and therefore the problems and/or disadvantages occurring in the previously proposed device shown in FIGS. 1 and 2 cannot be surely eliminated.